Commonly, roof frames are formed by a number of trusses placed at a slight distance from one another and extending from one facade wall to another, on which they bear. The roofing bears on the trusses, which are held in place by cross-pieces.
The trusses in principle comprise a horizontal beam called a tie-beam, which thus connects the facade walls at the front and the rear, and two inclined pieces of wood called the principal rafters, each one passing from one end opposite the entrance and joining each other at the apex of the roof. However, these trusses are light and have to be reinforced.
Their reinforcements consist of a triangulation formed of elements, some of which, operating under tension from the apex of the truss, support the tie-beam, while the others, operating under compression by bearing at the base of the former elements, support--generally at their center--the principal rafters, which otherwise would be too flexible to support the roofing. This triangulation, which is repeated at each truss and constitutes an impediment to the fitting out of the roof, evidently cannot be removed without endangering the principal rafters, the cladding, and the ceiling, without other precautions being taken.
In new constructions (French Pat. No. A-2411283), before cladding, it is, of course, known and simple to provide beams which extend from one gable wall to another and on which the principal rafters of the trusses bear, which may then be devoid of support triangulation, at least in the zone liable to form the habitable part of the roof.
When existing constructions are converted it is evidently possible to expose the frame so as to enable the passage and positioning of beams, as well as to adjust the trusses, but this is a very large and expensive job which, in addition, exposes the other parts of the construction to bad weather for a certain time.
In order to enable the triangulation to be withdrawn without removing the roofing, it is known to place support pillars beforehand under the principal rafters. Unfortunately these pillars cannot bear on the tie-beam which is not designed to take up such a load, instead they therefore have to go down as far as the first sufficiently strong slab, for example of concrete, which they meet in the construction and which generally constitutes the floor of the last storey of the building, in the rooms of which storey these pillars cannot always be easily concealed by panels or partitions.